1. Field of the Invention
The present invention relates to a method of preparing water-dispersible poly (urethane-urea) having aliphatic-aromatic isocyanate, more particularly to a method of preparing water-dispersible poly (urethane-urea) having aliphatic-aromatic isocyanate, with improved mechanical/thermal properties, such as heat resistance, chemical resistance and adhesion, good cost competitiveness and environmental friendliness, by reacting polyester polyol, dimethylol propionic acid and an aromatic isocyanate compound to obtain a hydroxyl (OH)-terminated prepolymer; adding an aliphatic isocyanate compound to the OH-terminated prepolymer to obtain an isocyanate (NCO)-terminated prepolymer wherein aromatic-aliphatic isocyanate is hybridized; neutralizing; water-dispersing and chain-extending the NCO-terminated prepolymer.
2. Description of the Related Art
Polyurethane is prepared into a variety of products, including hard foam, soft foam, film, soft/hard elastomer and fiber. Until the 1980s, the Korean polyurethane industry has developed focusing on daily goods materials such as clothes, shoes, bags, toys, sofas and beds. Currently, it has expanded its market to heavy industry materials for automobiles, electronics, civil engineering and shipbuilding.
Most of the conventional polyurethane products were organic solvent-based ones. However, with the rise of environmental recognition, demand on water-dispersible polyurethane is slowly increasing in the western world.
Efforts have been made to obtain water-dispersible polyurethane. Of them is a method of dispersing self-emulsifying resin, prepared by introducing hydrophilic groups to the polyurethane backbone, in water [Japanese Patent Publication Nos. Hei 4-328187 and Hei 5-43642], and a method of dispersing hydrophobic polyurethane by mechanical force using external emulsifying agents [Japan Patent Publication Nos. Sho 39-5989 and Sho 45-10957].
In the conventional preparation method of water-dispersible poly (urethane-urea), low-molecular-weight prepolymer with low viscosity having ionic groups is dispersed in water and a polymer dispersion solution is obtained from it using a chain extender, as follows.
Polyol and catalyst (if necessary) are sufficiently stirred at 80 to 100° C., and isocyanate is added. Reaction is carried out until the NCO group content of the isocyanate reaches the theoretical value. After adding dimethylol propionic acid and diols, reaction is carried out until the theoretical value is attained. Then, such solvent as acetone and methyl ethyl ketone is added to adjust viscosity. When an aliphatic isocyanate has been used, the NCO-terminated prepolymer is dispersed in water to form stable particles and the chain extension is carried out for 3 to 4 hours by adding an amine based chain extender. When an aromatic isocyanate has been used, coagulation or gelation may occur during the dispersion due to the side reaction of NCO group with water molecules. Therefore, the NCO groups should be consumed by using a diol or diamine based chain extender before the dispersion. For both cases, solvent remaining in the resultant dispersing solution can be removed by a vacuum rotary evaporator.
For an aliphatic isocyanate, a polyurethane resin with large molecular weight can be prepared by aqueous dispersion of the low-molecular-weight polyurethane prepolymer and followed by the chain extension. However, for an aromatic isocyanate, preparation of a polyurethane resin with large molecular weight is impossible, because chain extension is carried out in bulk prior to water-dispersing to prevent side reaction with water molecules.